In an autofocus (AF) control in a lens interchangeable type image pickup apparatus, as disclosed in Patent Literature 1 (PTL 1), a focus signal that indicates a contrast state of an image (a focus state of an image pickup optical system) is generated at a camera body side. Then, commonly, a correction lens (a focus lens) is moved based on the focus signal obtained from the camera body so as to perform the AF control. In this case, the focus signal according to characteristics of amounted lens unit is generated at the camera body side, and the AF control is performed at the lens unit side. Therefore, the accuracy of the focusing by the AF control is determined in accordance with the combination of the camera body and the lens unit.
On the other hand, recently, the performance of an image pickup element such as a CMOS sensor at the camera body side is dramatically improved, and the high accuracy is also required for the focusing according to this. In this case, it is preferred that the AF control be performed according to the performance of the image pickup element at the camera body side and that the correction lens be moved in accordance with a control signal supplied from the camera body at the lens unit.